How do I stop specific bones from participating in root motion?

I have a humanoid in Mecanim. The top three bones of its hierarchy are Root, Motion, and Hip (with everything attached to Hip as expected). The intent of this setup is:

• The Root bone never moves or rotates.
• Animating the Motion bone moves the character via root motion.
• Animating the Hip bone moves the body as necessary for the animation, but does not affect root motion.

I have a walk-up-an-incline animation where Motion moves up and forwards while Hip bobs up and down. The problem is that both of these bones are contributing towards the overall root motion - jittering the walk on and off makes the character slowly descend into the ground, for example. I need to have some way of turning off Hip's influence, or even better, explicitly saying "only the Motion bone should have any effect on root motion".

I can't just disable Y-movement by ticking the "bake into pose" box, because that affects all bones including Motion.

 

Maybe, you could create a bone mask, and mask out the hip bone.